I think the Bison is quite a bit larger than the Beaver. The specifications say it has 10 inches more wheelbase, which much larger tires, and it weighs something like 1000lbs more (nearly double). It's a markedly bigger machine.
Having said that, if the mowing deck will fit between the front and rear tires, it's probably possible to "make it work." I think you'll be doing some fabricating to make the lift work and the drive mechanism the correct length and so on.
The 650G is quite a bit more powerful than your Beaver, so you may be stuck going pretty slow with it in order to have the power to cut with the 5 foot deck, but it may work just fine on lawns or what have you if it doesn't get too long and you don't use the high lift mulching blades.
Get some dimensions on the deck and from your tractor.